Sparky,
It is not clear that you appreciate the severity of the thermal problem posed by the arrangement of TWO turbos in close proximity to each other and the differential housing, all in a small and presumably an essentially unventilated compartment. Not to mention waste gates and associated exhaust tubulars. What Rex has described is true about the characteristics of their heat generation. You’ve got a real problem and little space in which to address it. I don’t want to be overly negative, but this issue is something that needs to be competently resolved due to the possible consequences. Which I am sure you are also interested in accomplishing.
Quilted aluminum (melting point about 1150F) foil that normally shields a catalytic converter at a few hundred degrees while in a free flowing air stream is not going to solve the problem, although it may have a place in a more elaborate overall system, protecting surrounding features.
Oil, that cools an engine, again at a couple hundred degrees, is going to be unsatisfactory at anything over those temperatures. See Rex’s reference to “coal”.
Ceramic coating may be good for looks and preserving the surface of the turbine housing, but will have negligible effect on heat transfer rate due to the thinness of the coat.
Purpose-built turbo blankets may go a long way towards containing some of the heat, but with two together and essentially no cooling, they will likely produce a considerable heat-soak problem for the center sections, especially on shutdown, as well as for everything around them.
Mr. Borman is right with the passive cooling prescription--otherwise, what happens when the water spray/circulator quits working? It will need a lot of air, both while at speed and at rest.